JP5586152B2 - Polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a separation membrane for separation of substances, selective permeation, etc., and a microporous membrane widely used as a separator for electrochemical reaction devices such as alkali, lithium secondary batteries, fuel cells, capacitors, etc. The present invention relates to a polyolefin microporous membrane suitably used as a battery separator.

Polyolefin microporous membranes are widely used as separators for various substances, selective permeation separation membranes, separators, etc. Examples of applications include microfiltration membranes, fuel cell separators, capacitor separators, or functional materials For example, a functional film base material for filling the inside of a hole to cause a new function to appear, and a battery separator. In these applications, it is particularly preferably used as a separator for lithium ion batteries widely used in notebook personal computers, mobile phones, digital cameras and the like. The reason for this is that the membrane has mechanical strength and pore blocking properties.
Porosity is a performance that ensures the safety of the battery by melting and closing the hole when the battery is overheated in an overcharged state, etc., and blocking the battery reaction. The lower the temperature, the higher the safety effect.
Also, in order to prevent a short circuit due to foreign matter in the battery when winding the separator, the piercing strength and length direction of the separator (referred to as machine direction, hereinafter also referred to as MD), width direction (perpendicular to the machine direction) The tensile strength of TD) is required to have a certain level of strength. In addition, in recent lithium ion secondary batteries, in order to increase battery output and capacity, separators need to have not only large pore diameters but also excellent heat shrinkability at high temperatures. is there.
It is said that the separator has a higher porosity and larger pore diameter, the better the electric characteristics of the battery. However, the increase in porosity and the increase in the diameter of the pores have a contradictory relationship with the size and strength of the thermal shrinkage. For this reason, separators with high porosity and large pore diameter have problems that even if the battery electrical characteristics are good, the shrinkage is large or the strength is insufficient under the high temperature of the battery oven test.

As means for solving these problems, the present applicant has proposed a method of Patent Document 1 in which a polymer, a filler, and a plasticizer are kneaded and phase-separated, and subjected to stretching after extraction. As a result, a microporous membrane that has high porosity and large pore diameter but low heat shrinkage has been proposed, but it is difficult to achieve both low heat shrinkage while exhibiting sufficient strength in all directions in stretching after extraction. It is.
In addition, the present applicant has proposed a microporous membrane that is in a specific pore diameter range and has a specified water permeation amount / air permeation amount ratio through a specific extraction / stretching process in Patent Document 2. However, a membrane that has undergone such an extraction / stretching process tends to have a high thermal contraction rate, and the water permeation amount / air permeation amount described in this document has recently increased the output of lithium ions. In secondary batteries and the like, electrical characteristics tend to be insufficient.
In Patent Document 3, a high-molecular-weight polyolefin is used and a microporous membrane with a large pore diameter has been proposed, but it has reached a microporous membrane with excellent balance, such as a large pore diameter while having high heat resistance and high strength. Not in.
Further, Patent Document 4 proposes a microporous membrane having a high heat resistance and a large pore diameter. However, it is difficult to increase the strength of the membrane with such a manufacturing method. A microporous membrane with high strength has been proposed by a polyolefin blend, but since it is blended with low-density polyethylene, it becomes difficult to heat-set at a high temperature.

Japanese Patent No. 3258737 JP 2004-323820 A Japanese Patent Laid-Open No. 10-258462 Japanese Patent No. 3050021 JP-A-8-34873

  An object of the present invention is to provide a polyolefin microporous membrane that is excellent in strength and low heat shrinkability while having excellent electrical characteristics with a large pore size without degrading the properties of conventional polyolefin microporous membranes. .

As a result of intensive studies to achieve the above-mentioned object, the present inventors have found that the bubble point, the tensile strength in the length direction and the width direction, and the TD heat shrinkage rate at 130 ° C. are adjusted to a specific range. The present invention has been completed by finding that the microporous membrane has a large pore diameter and is excellent in strength and low heat shrinkability. That is, the present invention is as follows.
(1) The bubble point is 1 MPa or less, the tensile strength in the length direction and the tensile strength in the width direction are each 50 MPa or more , and the TD heat shrinkage rate at 130 ° C. measured by the following method A is 20% or less. Ri, viscosity-average molecular weight of entire polymer material is a polyolefin microporous membrane Ru der 300,000 over 800,000 or less,
A polyolefin microporous membrane, wherein the polyolefin is polyethylene or a blend of polyethylene and polypropylene .
[Method A]
It was cut to 100 mm in MD and 100 mm in TD, and left in an oven at 130 ° C. for 1 hour. At this time, it was sandwiched between two sheets of paper so that the warm air would not directly hit the sample. After taking out from the oven and cooling, the length (mm) was measured, and the TD heat shrinkage was calculated by the following formula (for samples where the sample length could not be secured, it was as long as possible within the range of 100 mm x 100 mm) sample.).
TD heat shrinkage rate (%) = (100−length of TD after heating) / 100 × 100
Here, MD means the length direction, and TD means the width direction.
(2) The polyolefin microporous membrane according to the above (1), wherein the ratio of water permeability / air permeability is 1.7 × 10 ⁻³ or more and less than 2.3 × 10 ⁻³ .
(3) The polyolefin microporous membrane according to (1) or (2), wherein the total of MD tensile elongation and TD tensile elongation is 20 to 250%.
(4) The polyolefin microporous membrane according to (1) or (2), wherein the total of MD tensile elongation and TD tensile elongation is 20 to 200%.
(5) The polyolefin microporous film according to any one of (1) to (4) above, comprising ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and polyethylene having a viscosity average molecular weight of less than 500,000.
(6) The polyolefin microporous membrane according to any one of (1) to (5), wherein the porosity is 20% or more and 60% or less.
(7) A battery separator comprising the polyolefin microporous film according to any one of (1) to (6) above.
(8) A non-aqueous electrolyte secondary battery comprising the battery separator according to (7) .

  The polyolefin microporous membrane of the present invention has a larger pore diameter than conventional polyolefin microporous membranes, and has excellent strength and low heat shrinkability. Therefore, it is possible to improve battery characteristics and battery safety by using the polyolefin microporous membrane of the present invention for a battery separator.

  Hereinafter, the best mode for carrying out the present invention (hereinafter also referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

  The polyolefin microporous membrane of the present embodiment has a bubble point of 1 MPa or less, a tensile strength in the length direction and a tensile strength in the width direction of 50 MPa or more, and a thermal contraction rate in the width direction at 130 ° C. of 20% or less. It is.

  The bubble point of the polyolefin microporous membrane needs to be 1.0 MPa or less, preferably 0.8 MPa or less, so that the pores do not become too dense. The lower limit of the bubble point is preferably 0.1 MPa or more, more preferably 0.3 MPa or more. If it is less than 0.1 MPa, the pores may be coarsened and the film strength may be lowered.

This bubble point method is known as a simple method for expressing the maximum pore size, but apart from the bubble point viewpoint, the water permeability of the microporous membrane, the ratio of the air permeability (water permeability / air permeability) is the average. There is a correlation with the typical pore size. This ratio is preferably 1.7 × 10 ⁻³ or more. If it is less than 1.7 × 10 ⁻³ , the permeability tends to be insufficient, and the capacity retention rate of the battery tends to decrease. The upper limit is not specified, but it is preferably less than 2.3 × 10 ⁻³ , more preferably less than 2.1 × 10 ⁻³ . If it is 2.3 × 10 ⁻³ or more, the pores may become too large and the strength may be insufficient, or a short circuit may occur due to lithium dendrite. When the bubble point is 1.0 MPa or less and the water permeability / air permeability ratio is in the above range, the average pore diameter balance is excellent, and it is easy to provide high strength and low heat shrinkage while maintaining permeability. In particular, it is particularly preferable because it brings good performance to the characteristics of recent lithium ion batteries.

  The polyolefin microporous membrane of the present embodiment needs to have a tensile strength in both the length direction (MD) and the width direction (TD) of 50 MPa or more, more preferably 70 MPa or more, and more preferably 100 MPa or more. preferable. If the tensile strength is weak (less than 50 MPa), the battery winding property is deteriorated, or a short circuit is likely to occur due to an external battery impact test, foreign matter in the battery, or the like.

  Furthermore, the microporous membrane made of polyolefin of the present embodiment has a heat shrinkage rate in the width direction (TD) at 130 ° C. of 20% or less, preferably 17% or less, from the viewpoint of ensuring safety in an oven test or the like. Preferably it is 15% or less. Although the heat shrinkage rate in the length direction (MD) at 130 ° C. is not particularly limited, it is preferably 20% or less, more preferably 17% or less, from the viewpoint of ensuring safety, as in the width direction. More preferably, it is 15% or less.

  The polyolefin microporous membrane of the present embodiment preferably contains polypropylene. By including polypropylene in the microporous membrane, not only can the heat resistance be improved, but it tends to be difficult to break even under a high draw ratio. Furthermore, it becomes easy to adjust the MD and TD tensile elongation described later to a suitable range, and as a result, the impact resistance of the obtained battery can be improved and the risk of short circuit can be reduced. The content of polypropylene is preferably 1 to 80% by mass, more preferably 2 to 50% by mass, and further preferably 3 to 30% by mass with respect to the polymer material. If it is less than 1% by mass, the effect tends to be difficult to be exhibited, and if it exceeds 80% by mass, the permeability tends to be difficult to ensure.

  Further, the polyolefin microporous membrane of the present embodiment preferably has an MD and TD tensile elongation of 10 to 200%, more preferably 10 to 150%, and more preferably 10 to 120%. More preferably. The total of MD tensile elongation and TD tensile elongation is preferably 20 to 250%, more preferably 20 to 230%, and still more preferably 20 to 200%. A microporous membrane having an MD and TD tensile elongation in the above range not only has good battery winding properties, but also makes it difficult for the wound body to undergo deformation in a battery impact test or the like. When the tensile elongation exceeds the above range, the elongation of the microporous membrane increases, and it tends to be deformed by repeated impacts in a battery impact test or the like, resulting in an increased risk of causing a short circuit.

  In order to obtain a microporous membrane having MD and TD tensile elongations in the above range, it is necessary to combine several methods. For example, the stretching ratio described later and various conditions in stretching and relaxation operations after extraction are adjusted. Can be achieved. As described above, mixing polypropylene in the polymer is also an effective method.

  The polyolefin microporous membrane of the present embodiment preferably contains ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and polyethylene having a viscosity average molecular weight of less than 500,000. By containing the above-mentioned various polyethylenes, not only the melt viscosity increases when the separator is melted, but also the tear resistance tends to be improved by early relaxation of the melt tension.

  The porosity of the polyolefin microporous membrane of the present embodiment is preferably 20% or more from the viewpoint of permeability, and 60% or less from the viewpoint of membrane strength, withstand voltage, and heat shrinkage. More preferably, they are 25% or more and 60% or less, More preferably, they are 30% or more and 55% or less.

  The air permeability of the polyolefin microporous membrane of the present embodiment is preferably as low as possible, but is preferably 1 sec or more, and more preferably 50 sec or more, from the viewpoint of the balance between thickness and porosity. Further, from the viewpoint of permeability, it is preferably 1000 sec or less, more preferably 500 sec or less.

  The thickness of the polyolefin microporous membrane of the present embodiment is preferably 1 μm or more, more preferably 5 μm or more from the viewpoint of membrane strength. Moreover, it is preferable that it is 50 micrometers or less from a viewpoint of permeability, and 30 micrometers or less are more preferable.

  Further, the puncture strength of the polyolefin microporous membrane of the present embodiment is preferably 0.2 N / μm or more, and more preferably 0.22 N / μm or more. When the puncture strength is low (less than 0.2 N / μm), when used as a battery separator, sharp parts such as electrode materials pierce the microporous film, and pinholes and cracks are likely to occur. It tends to be easily deformed in an external battery impact test or the like.

  Next, a method for producing a polyolefin microporous membrane according to the present embodiment will be described. If the resulting microporous membrane has the above characteristics, a polymer species, a solvent species, an extrusion method, a stretching method, and an extraction method. In the opening method, heat setting / heat treatment method and the like, there is no limitation.

  As a method for producing a polyolefin microporous membrane according to the present embodiment, a step of melt-kneading and extruding a resin composition containing at least polyolefin and a plasticizer to obtain a sheet-like material, and stretching the sheet-like material to form a film It is preferable to include the process of obtaining, the process of extracting a plasticizer from the said sheet-like material or the said film, and the process of heat-setting the said film.

The polyolefin microporous membrane of the present embodiment is obtained, for example, by a method comprising the following steps (a) to (e).
(A) Dissolving and kneading any polymer material of a single polyolefin, a polyolefin mixture, a polyolefin solvent mixture and a polyolefin kneaded product.
(B) The melted material is extruded, formed into a sheet, and cooled and solidified. If necessary, a plasticizer and an inorganic agent are extracted.
(C) The obtained sheet is stretched in a direction of one axis or more.
(D) After stretching, a plasticizer and an inorganic agent are extracted as necessary.
(E) Next, heat setting and heat treatment are performed.

  The polyolefin used in this embodiment is a homopolymer of ethylene or propylene, or a copolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene, norbornene In addition, a mixture of the above polymers may be used. Among these, polyethylene and copolymers thereof are preferable from the viewpoint of improving the performance of the microporous membrane. Examples of such polyolefin polymerization catalysts include Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, and the like. The polyolefin may be obtained by a one-stage polymerization method or may be obtained by a multi-stage polymerization method.

  As the composition of the polymer to be supplied, it is preferable to blend two or more kinds of polyolefins. This makes it possible to control the fuse temperature and the short-circuit temperature. More preferably, two or more types of polyethylene are blended, and it is preferable to include an ultrahigh molecular weight polyethylene having a viscosity average molecular weight (Mv) of 500,000 or more and a polyethylene having a viscosity average molecular weight (Mv) of less than 500,000. . The polyethylene to be blended is preferably a high-density homopolymer from the viewpoint that heat setting can be performed at a higher temperature without clogging the pores.

  Moreover, it is preferable that the viscosity average molecular weights (Mv) of the whole polymer material are 100,000 or more and 1.2 million or less. More preferably, it is 300,000 or more and 800,000 or less. If the viscosity average molecular weight (Mv) is less than 100,000, the film-breaking resistance at the time of melting may be insufficient, and if it exceeds 1,200,000, the extrusion process becomes difficult, the relaxation of the shrinkage force at the time of melting is slow, and heat resistance May be inferior.

  Blending these with polypropylene, which is a polyolefin having a higher melting point than polyethylene, not only increases heat resistance, but also enables operation at higher temperatures than polyethylene alone in the stretching / relaxation process after extraction, It is possible to reduce the tensile elongation while maintaining the strength, heat shrinkage rate, and pore diameter of the microporous membrane. Furthermore, although the reason is not clear, it is particularly preferable because it has an effect that it is difficult to break even under a high draw ratio.

  The improvement in heat resistance by blending as described above is preferable because the film resistance at high temperatures becomes better by combining with the low heat shrinkability of the present application.

  Also, known additives such as metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.

  Furthermore, inorganic agents such as alumina and titania can be added. The inorganic agent may be extracted in whole or in part in any of the entire steps, or may remain in the product.

  The plasticizer used in the present embodiment is an organic compound that can form a uniform solution with polyolefin at a temperature below the boiling point, specifically decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol. Oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, paraffin oil and the like. Of these, paraffin oil and dioctyl phthalate are preferred.

  Although the ratio of a plasticizer is not specifically limited, 20 mass% or more is preferable from a viewpoint of the porosity of the microporous film obtained, and 90 mass% or less is preferable from a viscosity viewpoint. More preferably, it is 50 mass% or more and 70 mass% or less.

  The extraction solvent used for the extraction of the plasticizer is preferably a poor solvent for the polyolefin, a good solvent for the plasticizer, and a boiling point lower than the melting point of the polyolefin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbons, alcohols such as ethanol and isopropanol, and acetone. And ketones such as 2-butanone. It selects from these and uses it individually or in mixture. These extraction solvents may be regenerated by distillation after extraction of the plasticizer and used again.

  The total weight ratio of the plasticizer and the inorganic agent in the entire mixture to be melt-kneaded is preferably 20 to 95% by mass and more preferably 30 to 80% by mass from the viewpoint of membrane permeability and film-forming property.

  Moreover, it is preferable to mix | blend antioxidant in a mixture from a viewpoint of preventing the heat deterioration at the time of melt-kneading, and the quality deterioration by it. The concentration of the antioxidant is preferably 0.3% by mass or more, and more preferably 0.5% by mass or more with respect to the total polyolefin weight. Moreover, 5.0 mass% or less is preferable and 3.0 mass% or less is more preferable.

  As the antioxidant, a phenolic antioxidant which is a primary antioxidant is preferable, and 2,6-di-t-butyl-4-methylphenol, pentaerythrityl-tetrakis- [3- (3,5- Di-t-butyl-4-hydroxyphenyl) propionate], octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, and the like. Secondary antioxidants can also be used in combination, such as tris (2,4-di-t-butylphenyl) phosphite, tetrakis (2,4-di-t-butylphenyl) -4,4- Examples thereof include phosphorus-based antioxidants such as biphenylene-diphosphonite and sulfur-based antioxidants such as dilauryl-thio-dipropionate.

  As a method of melt kneading and extrusion, first, a part or all of raw materials are premixed by a Henschel mixer, a ribbon blender, a tumbler blender, or the like as necessary. If the amount is small, it may be stirred by hand. Next, all the raw materials are melt-kneaded by a screw extruder such as a single screw extruder or a twin screw extruder, a kneader, a mixer or the like, and extruded from a T-die or an annular die.

  The melt-kneading is preferably performed in a state where the raw material polymer is mixed with an antioxidant at a predetermined concentration and then replaced with a nitrogen atmosphere and the nitrogen atmosphere is maintained. The temperature during melt kneading is preferably 160 ° C. or higher, and more preferably 180 ° C. or higher. Moreover, less than 300 degreeC is preferable, less than 240 degreeC is more preferable, and less than 230 degreeC is further more preferable.

  The melt referred to in the present embodiment may include an unmelted inorganic agent that can be extracted in the inorganic agent extraction step. In addition, the melted and kneaded and homogenized melt is preferably passed through a screen to improve film quality.

  Next, the formation of the gel sheet will be described. As a method for forming the gel sheet, it is preferable to solidify the melted and kneaded and extruded melt by compression cooling. Examples of the cooling method include a method of directly contacting a cooling medium such as cold air or cooling water, a method of contacting a roll or press machine cooled with a refrigerant, and a method of contacting a roll or press machine cooled with a refrigerant. , Which is preferable in terms of excellent thickness control.

  The stretching, plasticizer extraction, or stretching, plasticizer extraction, and inorganic agent extraction, which are subsequently performed, are not particularly limited in order, method, and number of times. The inorganic agent extraction may not be performed as necessary.

  As stretching methods, MD uniaxial stretching using a roll stretching machine, TD uniaxial stretching using a tenter, sequential biaxial stretching using a roll stretching machine and a tenter, or a combination of a tenter and a tenter, simultaneous biaxial stretching using a simultaneous biaxial tenter or inflation molding. Etc. The draw ratio is a total area ratio, and in order to obtain desired tensile strength and tensile elongation, it is preferably 8 times or more, more preferably 15 times or more, further preferably 30 times or more, and particularly preferably 40 times or more. Among these, simultaneous or sequential biaxial stretching is preferable. Further, for the same reason, the total draw ratio in all steps is preferably 50 times or more, more preferably 60 times or more.

  In the plasticizer extraction, the plasticizer is extracted by dipping or showering in an extraction solvent. Then, it is sufficiently dried.

  As a heat setting method, a relaxation operation is performed in a predetermined temperature atmosphere and a predetermined relaxation rate. It can be performed using a tenter or a roll stretching machine. The relaxation operation is an operation for reducing the film to MD and / or TD. The relaxation rate is a value obtained by dividing the MD dimension of the film after the relaxation operation by the MD dimension of the film before the operation, or a value obtained by dividing the TD dimension after the relaxation operation by the TD dimension of the film before the operation, or MD, TD When both are relaxed, it is a value obtained by multiplying the MD relaxation rate and the TD relaxation rate. The predetermined relaxation rate is preferably 0.9 or less, and more preferably 0.8 or less, from the viewpoint of the heat shrinkage rate. Moreover, it is preferable that it is 0.6 or more from a viewpoint of wrinkle generation | occurrence | production prevention, a porosity, and permeability | transmittance. The relaxation operation may be performed in both the MD and TD directions. However, even with the relaxation operation of only one of the MD and TD, it is possible to reduce the thermal contraction rate not only in the operation direction but also in the operation and the vertical direction. By subjecting the film to stretching 1.5 times or more, more preferably 1.8 times or more before this relaxation operation, it becomes easy to obtain a microporous membrane having a high strength and a large pore diameter.

  The stretching and relaxation operations after the plasticizer extraction are preferably performed in the TD direction. From the viewpoints of heat shrinkage and increase in pore size, the temperature in the stretching step before the relaxation operation and the relaxation operation is preferably 125 ° C or higher, and at least one of them is preferably 130 ° C or higher, more preferably 132 ° C. It is above ℃. When the temperature in the stretching step before the relaxation operation and the relaxation operation is in the above range, when the polyolefin is polyethylene, the stretching / relaxation operation is performed in the vicinity of the melting point, which has a larger pore diameter and lower heat than conventional microporous membranes. A product with a shrinkage rate is easily obtained. Furthermore, although the reason is not clear, it is easy to obtain a microporous film having excellent film breaking property even with a low elongation film. Polypropylene is blended as a polyolefin in addition to polyethylene from the viewpoint that it can be stretched and relaxed under higher temperature conditions, which are different from conventional ones, and it is difficult to break even under conditions where the total draw ratio is large. It is preferable.

  In addition, the polyolefin microporous film of the present embodiment can be subjected to a surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, or chemical modification.

Hereinafter, the present embodiment will be described in more detail with reference to examples.
[Measuring method]
The measuring method of the physical property etc. in this specification is as follows.
(1) Viscosity average molecular weight (Mv)
Based on ASTM-D4020, the intrinsic viscosity [η] at 135 ° C. in a decalin solvent is determined. Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
For polypropylene, Mv was calculated by the following formula.
[Η] = 1.10 × 10 ⁻⁴ Mv ^0.80

(2) Film thickness (μm)
The measurement was performed at a room temperature of 23 ± 2 ° C. using a fine thickness measuring instrument manufactured by Toyo Seiki, KBM (trademark).

(3) Porosity (%)
A sample of 10 cm × 10 cm square was cut from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated from the material density.

(4) Air permeability (sec)
In accordance with JIS P-8117, a Gurley type air permeability meter (G-B2 (trademark), manufactured by Toyo Seiki Co., Ltd.) was used. The inner cylinder weight was 567 g, and the time required for 100 ml of air to pass through an area of 28.6 mm in diameter and 645 mm ² was measured.

(5) Air permeability The air permeation rate constant Rgas was determined from the air permeability (sec) using the following equation. The measurement was carried out indoors at room temperature 23 ° C.
Rgas (m ³ / (m ² · sec · Pa)) = 0.0001 / air permeability / 0.0006424 / (0.01276 × 101325)

(6) Water permeability A microporous membrane previously immersed in alcohol is set in a stainless steel permeation cell having a diameter of 41 mm, and after the alcohol in the membrane is washed with water, water is allowed to permeate at a differential pressure of about 50000 Pa. The water permeation amount per unit time, unit pressure, and unit area was calculated from the water permeation amount (cm ³ ) when 120 seconds passed, and this was defined as the water permeability (cm ³ / (cm ² · sec · Pa)). . The measurement was carried out indoors at room temperature 23 ° C. The water permeation rate constant Rliq was determined from the water permeability (cm ³ / (cm ² · sec · Pa)) using the following equation.
Rliq (m ³ / (m ² · sec · Pa)) = water permeability / 100

(7) Puncture strength (N / μm)
Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, the microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the central portion of the fixed microporous membrane is subjected to a puncture test in a 25 ° C. atmosphere at a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. / Puncture strength (N / μm) multiplied by film thickness (μm) was calculated.

(8) Tensile strength (MPa), tensile elongation (%)
Based on JIS K7127, it measured about MD and TD sample (shape; width 10mm x length 100mm) using the tensile tester by Shimadzu Corporation, and autograph AG-A type (trademark). Moreover, the sample used the thing which stuck the cellophane tape (the Nitto Denko Packaging System Co., Ltd. make, brand name: N.29) to the single side | surface of the both ends (each 25mm) of the sample between chuck | zippers 50mm. Furthermore, in order to prevent sample slipping during the test, 1 mm-thick fluororubber was affixed inside the chuck of the tensile tester.
The tensile elongation (%) was obtained by dividing the amount of elongation (mm) up to fracture by the distance between chucks (50 mm) and multiplying by 100.
The tensile strength (MPa) was obtained by dividing the strength at break by the sample cross-sectional area before the test.
Moreover, the sum (%) of MD tensile elongation and TD tensile elongation was calculated | required by totaling the value of MD and TD. The measurement was performed at a temperature of 23 ± 2 ° C., a chuck pressure of 0.30 MPa, and a tensile speed of 200 mm / min (in the case of a sample where the distance between chucks cannot be secured 50 mm, the strain rate is 400% / min).

(9) 130 ° C. heat shrinkage (%)
It was cut to 100 mm in the MD direction and 100 mm in the TD direction, and left in an oven at 130 ° C. for 1 hour. At this time, it was sandwiched between two sheets of paper so that the warm air would not directly hit the sample. After taking out from the oven and cooling, the length (mm) was measured, and the MD thermal contraction rate and TD thermal contraction rate were calculated by the following equations. (For samples for which the sample length cannot be secured, the sample is as long as possible within the range of 100 mm × 100 mm.)
MD thermal shrinkage (%) = (100− MD length after heating) / 100 × 100
TD heat shrinkage rate (%) = (100−length of TD after heating) / 100 × 100

(10) Bubble point (MPa)
Based on ASTM F316-86, measurement was performed with an ethanol solvent. The point at which continuous bubbles were confirmed was defined as the bubble point.

(11) High-speed thermal film resistance (film resistance)
10 μm thick nickel foil A (length 100 mm × width 25 mm), nickel foil B (length 100 mm × width 15 mm), separator immersed in electrolyte for 30 minutes or more (MD length 75 mm × TD length 25 mm), center Aramica film (trademark) provided with a 10 mm × 10 mm window, a slide glass (length 75 mm × width 25 mm), and a glass plate (length 25 mm × width 20 mm) are prepared.
As shown in FIG. 1, slide glass, nickel foil A, separator, aluminum film, nickel foil B, and glass plate were stacked in this order and fixed with clips.
A thermocouple was connected to the cell and left in the oven. Thereafter, the temperature was raised at a rate of 5 ° C./min. After reaching 150 ° C., holding was performed at 150 ° C. for 1 hour. The change in impedance at this time was measured with an LCR meter under conditions of AC 10 mV and 1 kHz. In this measurement, from the time when the impedance was held at 150 ° C., A was able to maintain an insulation state of 60 Ω or more and 1000 Ω or more, B was able to hold for 30 minutes or more, B was able to hold 10 minutes or more, A sample that could be held for 5 minutes or more was designated D, and a sample that could not be held for 5 minutes was designated E.
The composition ratio of the prescribed electrolyte solution was as follows.
Composition ratio (volume ratio) of solvent: propylene carbonate / ethylene carbonate / γ-butyllactone = 1/1/2
Solute composition ratio: LiBF ₄ was dissolved in the above solvent to a concentration of 1 mol / liter.

[Production and evaluation of batteries]
(1) Production of positive electrode 92.2% by mass of lithium cobalt composite oxide LiCoO ₂ as an active material, 2.3% by mass of flake graphite and acetylene black as a conductive agent, and polyvinylidene fluoride (PVDF) 3 as a binder A slurry was prepared by dispersing 2% by mass in N-methylpyrrolidone (NMP). This slurry was applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode was 250 g / m ² , and the active material bulk density was 3.00 g / cm ³ . This was cut into a width of about 40 mm to form a strip.
(2) Production of negative electrode 96.9% by mass of artificial graphite as an active material, 1.4% by mass of ammonium salt of carboxymethylcellulose and 1.7% by mass of styrene-butadiene copolymer latex as a binder were dispersed in purified water to form a slurry. Was prepared. This slurry was applied to one side of a 12 μm thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the negative electrode was set to 106 g / m ² , and the active material bulk density was set to 1.35 g / cm ³ . This was cut into a width of about 40 mm to form a strip.
(3) Preparation of Nonaqueous Electrolytic Solution Prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / liter. (4) Battery assembly A separator using a microporous membrane made of polyolefin, a strip-like positive electrode, and a strip-like negative electrode are stacked in the order of the strip-like negative electrode, the separator, the strip-like positive electrode, and the separator, and wound in a spiral shape to make an electrode plate laminate. Produced. This electrode plate laminate is pressed into a flat plate shape, and then stored in an aluminum container. The aluminum lead is led out from the positive electrode current collector to the battery lid, and the nickel lead is led out from the negative electrode current collector to the bottom of the container. Welded. Further, the non-aqueous electrolyte described above was poured into this container and sealed. The lithium ion battery thus produced was designed to have a length (thickness) of 6.3 mm, a width of 30 mm, a height of 48 mm and a nominal discharge capacity of 620 mAh.
(5) Battery evaluation (at 25 ° C atmosphere)
The lithium ion battery assembled as described above was charged with a constant current and constant voltage (CCCV) for 6 hours under the conditions of a current value of 310 mA (0.5 C) and a final battery voltage of 4.2 V. At this time, the current value immediately before the end of charging was almost zero. Then, it was left (aged) for 1 week in an atmosphere at 25 ° C.
Next, the battery was charged at a constant current and constant voltage (CCCV) for 3 hours under the conditions of a current value of 620 mA (1.0 C) and a termination battery voltage of 4.2 V, and discharged to a battery voltage of 3.0 V at a constant current value (CC) of 620 mA. The cycle of doing. The discharge capacity at this time was defined as the initial discharge capacity.
(A) Further, the above cycle was repeated 300 times. In this cycle, the ratio (%) of the capacity at the 300th cycle to the initial discharge capacity was defined as the capacity maintenance rate. A high capacity retention rate means good cycle characteristics.
(B) Separately, for the impact test of the battery before the cycle test of (a), the battery was repeatedly dropped 10 times from a height of 1.9 m onto the concrete floor. Thereafter, the battery was disassembled and observed. A specimen in which almost no deformation of the wound body was observed was designated as A, a specimen that was slightly seen was designated as B, and a specimen in which the deformation was easily confirmed was designated as C.

[Example 1]
Using a tumbler blender, 47% by mass of homopolymer polyethylene having an Mv of 700,000, 46% by mass of homopolymer polyethylene having an Mv of 300,000, and 7% by mass of polypropylene having an Mv of 400,000 were dry blended using a tumbler blender. 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added to 99% by mass of the obtained pure polymer mixture, and the tumbler was again added. A dry blend was performed using a blender to obtain a polymer mixture. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.
The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 65% by mass. The melt kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h.
Subsequently, the melt-kneaded material was extruded and cast on a cooling roll controlled at a surface temperature of 25 ° C. through a T-die, thereby obtaining a gel sheet having a thickness of 2000 μm.
Next, it led to the simultaneous biaxial tenter stretching machine, and biaxial stretching was performed. The set stretching conditions were an MD magnification of 7.0 times, a TD magnification of 7.0 times, and a preset temperature of 125 ° C.
Next, the solution was introduced into a methyl ethyl ketone tank and sufficiently immersed in methyl ethyl ketone to extract and remove liquid paraffin, and then the methyl ethyl ketone was removed by drying.
Next, it was led to a TD tenter and heat fixed. The stretching temperature and magnification during heat setting were 128 ° C. and 2.0 times, and the temperature and relaxation rate during the subsequent relaxation were 133 ° C. and 0.80.
Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 2]
It carried out similarly to Example 1 except the biaxial stretching temperature being 120 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 3]
The thickness of the original fabric after extrusion is 900 μm, the biaxial stretching temperature is 122 ° C., the stretching temperature and magnification during heat setting are 130 ° C. and 2.0 times, and the temperature and relaxation rate during the subsequent relaxation are 135 ° C., 0 The same procedure as in Example 1 was performed except that. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 4]
Using 30% by mass of homopolymer polyethylene with an Mv of 2.5 million and 70% by mass of homopolymer polyethylene with an Mv of 250,000, the original thickness after extrusion is 2400 μm, and the stretching temperature and magnification at the time of heat setting are The same procedure as in Example 1 was performed except that the temperature and relaxation rate during the subsequent relaxation were set at 132 ° C. and 0.7 at 125 ° C. and 1.9 times. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 5]
The same procedure as in Example 1 was performed except that homopolymer polyethylene having an Mv of 500,000 was used for 99% by mass of the pure polymer mixture, and the stretching temperature during heat setting was 125 ° C. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 6]
After extruding, the original thickness is 1800μm, the biaxial stretching ratio is 5 × 5 times, the biaxial stretching temperature is 115 ° C, and the stretching temperature and magnification at the time of heat setting are 125 ° C and 1.7 times. Was performed in the same manner as in Example 4 except that the temperature and relaxation rate were set to 131 ° C. and 0.70. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 7]
The same operation as in Example 5 was performed except that homopolymer polyethylene having an Mv of 1,200,000 was used and the biaxial stretching temperature was set to 128 ° C. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 8]
Using a blend of 45% by mass of homopolymer polyethylene with an Mv of 700,000, 40% by mass of homopolymer polyethylene with an Mv of 300,000, and 15% by mass of polypropylene with an Mv of 400,000, and a biaxial stretching temperature of 123 ° C. Was carried out in the same manner as in Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 9]
Using a blend of 45% by mass of homopolymer polyethylene with an Mv of 700,000, 30% by mass of homopolymer polyethylene with an Mv of 300,000, and 25% by mass of polypropylene with an Mv of 400,000, and a biaxial stretching temperature of 123 ° C. Was carried out in the same manner as in Example 1. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 10]
The same procedure as in Example 1 was performed except that the gel sheet thickness was 1600 μm, the stretching temperature during heat setting was 125 ° C., and the temperature during subsequent relaxation was 130 ° C. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Example 11]
30% by mass of homopolymer polyethylene with Mv of 2.5 million, 60% by mass of homopolymer polyethylene with Mv of 250,000, and 10% by mass of polypropylene with Mv of 400,000. It carried out similarly to Example 4 except having set it as 128 degreeC and 133 degreeC. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative Example 1]
The same procedure as in Example 1 was performed except that the stretching temperature and magnification during heat setting were 120 ° C. and 1.5 times, and the temperature and relaxation rate during the subsequent relaxation were 125 ° C. and 0.80. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative Example 2]
The same procedure as in Example 2 was conducted except that the stretching temperature and magnification during heat setting were 122 ° C. and 1.3 times, and no relaxation was performed. Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

[Comparative Example 4]
30% by mass of homopolymer polyethylene with an Mv of 700,000, 15% by mass of homopolyethylene with an Mv of 300,000, 5% by mass of homopolymer polypropylene with an Mv of 400,000, 30.6% by mass of dioctyl phthalate (DOP) %, 18.4% by mass of fine silica, 1% by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant and mixed. did. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere.
The melt kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 240 rpm, and a discharge rate of 12 kg / h.
Subsequently, the melt-kneaded material was extruded and cast on a cooling roll controlled at a surface temperature of 80 ° C. through a T-die, thereby obtaining a gel sheet having a thickness of 110 μm.
DOP and fine silica were extracted and removed from this gel sheet to obtain a microporous membrane. Two microporous membranes were stacked and stretched 5 times in the longitudinal direction at 110 ° C., and then led to a TD tenter and stretched 2 times in the lateral direction at 130 ° C. Thereafter, the TD relaxation rate was set to 0.80 at 130 ° C.
Table 1 shows the physical properties of the obtained polyolefin microporous membrane.

From the results in Table 1, the following can be understood.
(1) The polyolefin microporous membranes of Examples 1 to 11 in which the bubble point, the tensile strength in the length direction and the width direction, and the thermal shrinkage in the width direction at 130 ° C. were adjusted to a specific range, The balance of the film breaking property was good, and the battery produced using this had an excellent capacity retention rate.
(2) The polyolefin microporous membranes of Comparative Examples 1 and 2 had an inferior capacity retention rate because their bubble points exceeded 1 MPa and they did not have a sufficient pore size.
( 4) The microporous membrane made of polyolefin of Comparative Example 4 has a TD tensile strength of less than 50 MPa, and the sum of MD tensile elongation and TD tensile elongation exceeds 250%. The microporous membrane was easily deformed and had poor impact resistance.
(5) Compared with Examples 5 and 7, the polyolefin microporous membrane of Example 11 is blended with polyethylene having an Mv of 500,000 or more, polyethylene with less than 500,000, and further polypropylene, so Both film properties and impact resistance were excellent.
(6) The polyolefin microporous membranes of Examples 8 and 9 have a higher polypropylene content than Example 1. Therefore, not only was heat fixation possible at a high temperature, but also the tensile elongation was low, and both the film resistance and the impact resistance were excellent.
(7) Compared with Example 6, the polyolefin microporous membrane of Example 4 was superior in impact resistance because of its high total draw ratio and low tensile elongation.
(8) The polyolefin microporous membrane of Example 1 has a lower porosity and a lower capacity retention rate than Example 10, but has a high strength and a low thermal shrinkage rate. Excellent impact resistance.
From the above results, the polyolefin microporous membrane of the present embodiment had a strength, tensile elongation, and low heat shrinkability with excellent balance while having a large pore diameter. Therefore, by using the polyolefin microporous membrane of the present embodiment for a battery separator, a secondary battery having an excellent balance between battery characteristics and battery safety can be obtained.

  The present invention relates to a polyolefin microporous membrane used for separation of substances, permselective separation membranes, separators, and the like, and has industrial applicability particularly as a separator used in lithium ion batteries and the like.

Sectional drawing of the cell used for the measurement test of high-speed thermal film-breaking property (film-breaking resistance) is shown. DESCRIPTION OF SYMBOLS 1 ... Separator; 2 ... Nickel foil A; 3 ... Nickel foil B; 4 ... Aramika film; 5 ... Glass plate; 6 ... Slide glass; ; 8 ... 10x10mm window

Claims (8)

The bubble point is 1 MPa or less, the tensile strength in the length direction and the tensile strength in the width direction are each 50 MPa or more, and the TD heat shrinkage at 130 ° C. measured by the following method A is 20% or less. A polyolefin microporous membrane having an overall viscosity average molecular weight of 300,000 to 800,000,
A polyolefin microporous membrane, wherein the polyolefin is polyethylene or a blend of polyethylene and polypropylene.
[Method A]
Cut to 100 mm in MD and 100 mm in TD , it was left in an oven at 130 ° C. for 1 hour. At this time, it was sandwiched between two sheets of paper so that the warm air would not directly hit the sample. After taking out from the oven and cooling, the length (mm) was measured, and the TD heat shrinkage was calculated by the following formula (for samples where the sample length could not be secured, it was as long as possible within the range of 100 mm x 100 mm) sample.).
TD heat shrinkage rate (%) = (100−length of TD after heating) / 100 × 100
Here, MD means the length direction, and TD means the width direction. 2. The polyolefin microporous membrane according to claim 1, wherein a ratio of water permeability / air permeability is 1.7 × 10 ⁻³ or more and less than 2.3 × 10 ⁻³ .   The polyolefin microporous membrane according to claim 1 or 2, wherein the total of MD tensile elongation and TD tensile elongation is 20 to 250%.   The polyolefin microporous film according to claim 3, wherein the total of MD tensile elongation and TD tensile elongation is 20 to 200%.   The polyolefin microporous film according to any one of claims 1 to 4, comprising ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 500,000 or more and polyethylene having a viscosity average molecular weight of less than 500,000.   The polyolefin microporous membrane according to any one of claims 1 to 5, having a porosity of 20% or more and 60% or less.   A battery separator comprising the polyolefin microporous membrane according to any one of claims 1 to 6.   A nonaqueous electrolyte secondary battery comprising the battery separator according to claim 7.

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